The application generally relates to a system and method of cooling compressor motors in vapor compression systems.
Hermetic motors may experience windage losses because of friction caused during rotation. Windage losses adversely impact motor performance and efficiency. To reduce windage losses in the motor, factors directly related to the motor, for example, the peripheral speed of the rotor, the flow and thermodynamic state conditions of motor cooling gas circulated around the motor, the rotor surface area and the roughness of the rotor surface may be controlled to reduce friction in the motor.
One method for reducing energy losses in motors while cooling the motor is by suctioning refrigerant toward the motor windings. The reduction in temperature caused by suctioning refrigerant across the motor windings prevents the motor components from overheating and increases motor operating efficiency. Another method for reducing energy losses in motors is to maintain a constant pressure throughout the motor cavity. A pressure valve can be placed within the motor cavity to release higher-pressure gas build up that occurs in the motor cavity during operation. As the pressure in the cavity increases, the valve opens, thereby releasing high-pressure gases. The maintenance of constant pressure in the cavity increases motor efficiency. However, this method uses mechanical equipment and is not optimal for maintaining a true constant pressure in the motor cavity. Additionally, this method does not address the issue of the motor cavity temperature.
An additional method controls energy losses in motors by maintaining a constant pressure in the motor cavity, while also preventing the oil losses between motor components. The preservation of oil in the motor bearing components allows for greater lubrication for the movement of parts thereby reducing friction while not allowing oil to escape into the motor cooling cavity, preventing excessive oil churning and reducing energy losses. A hermetically sealed housing containing the refrigeration compressor transmission and oil supply reservoir is connected to the suction side of the compressor to equalize the pressure in the housing. The focus of the method is to prevent the boiling of refrigerant from the oil reserve. However, this system only holds the pressure in the motor cavity at a constant level, and only assists in reducing energy losses, rather than optimizing the motor efficiency.
For very high speed motors however, windage losses can still be substantial even after factors such as the peripheral speed of the rotor, the density and flow of motor cooling gas around the motor, the rotor surface area and/or the roughness of the rotor surface are optimized. The only remaining factor that can be manipulated to reduce windage losses is the density of the gas in the motor cavity. Windage losses decrease as the density of the gas in the motor cavity decreases resulting in better motor efficiency.
To reduce the gas density in these higher-speed motor cavities, vacuum pumps are used to lower the pressure surrounding the motors to reduce windage losses as much as possible. However, the use of a vacuum pump does not provide the ability to both adequately cool the motor and provide a vacuum surrounding the motor cavity. One attempt to lower the gas density in the motor cavity while simultaneously cooling the motor involves the use of auxiliary positive displacement gas compressors powered by an independent power source to “pump down” the motor cavity while a complete vapor compression system is in operation. However the auxiliary compressors can consume more energy than is saved in motor windage losses.
Other conventional rotor cooling systems for hermetic/semi-hermetic motors in vapor compression systems rely on evaporator gas directed through the rotor and vented into the lowest pressure location at the impeller suction inlet to the compressor. The system is used to minimize the windage, or friction, loss of the rotor by maintaining the refrigerant density within the system near evaporator conditions. The windage loss in the motor is nearly directly proportional to the gas density in the motor cavity for a constant speed of the rotor.
A potentially undesirable result of using the lowest pressure gas for motor cooling to minimize motor losses is that the seal leakage in the compressor is actually maximized because the largest differential pressure across the seals are experienced. This argument applies to any seal vented through the motor cavity to first stage suction. The pressure upstream of the seal is at each respective impeller discharge static condition, and the downstream pressure is at motor cavity pressure—that is, near evaporator pressure—when utilizing evaporator vapor to cool the rotor. The system minimizes losses if the motor windage losses are the only consideration. However, by utilizing evaporator conditions for motor cooling, seal leakage in the compressor may increase, particularly within a two-stage compressor.